The present invention relates to a low temperature fired dielectric ceramic composition in which the dielectric constant does not alter from its base value by more than 15 percent over a wide temperature range. More specifically, this invention relates to a dielectric ceramic composition with a dielectric constant of over about 2400 which is formed by firing a base ceramic preparation comprising a mixture of dielectric oxides and a ceramic flux at temperatures not exceeding about 1150.degree. C.
Multilayer ceramic capacitors are commonly made by casting or otherwise forming insulating layers of dielectric ceramic powder, placing thereupon conducting metal electrode layers, usually in the form of a metallic paste, stacking the resulting elements to form the multilayer capacitor, and firing to densify the material and form a solid solution of the constituent dielectric oxides. Barium titanate is one of the dielectric oxides frequently used in the formation of the insulating ceramic layer. Because of the high Curie temperature of barium titanate, however, other oxides are commonly reacted with the barium titanate to form a solid solution, thereby reducing the Curie temperature of the resulting ceramic material. Because the dielectric constant is highest at the Curie temperature of a material, it is desirable that a material for use as a capacitor has a Curie temperature around room temperature. Certain other oxides, such as manganese dioxide may also be added to improve the insulation resistance and control the dielectric constant of the resulting material by acting as a grain growth controller.
The variation with temperature of the dielectric constant of a ceramic composition for use in a multi-layer capacitor is also of substantial importance. Many dielectric ceramic compositions, including barium titanate, have dielectric constants which vary substantially as the temperature increases or decreases. In a desirable dielectric ceramic composition for a multilayer capacitor used for applications requiring stability in the dielectric constant over a wide temperature range, the dielectric constant does not alter from its base value at 25.degree. C. (room temperature) by more than plus or minus 15 percent. As with the adjustment of the Curie temperature, reaction of the barium titanate with selected oxides is necessary to achieve this flat temperature characteristic.
The materials commonly used to produce such temperature stable capacitors with dielectric constants greater than 2000, are generally fired to maturity in air at temperatures greater than 1150.degree. C. At these temperatures, the metal electrode layer must be formed from the less reactive, higher melting alloys of the so-called precious metals, such as palladium and silver, palladium and gold, and other similarly expensive alloys well known in the art. This is necessary in order to prevent either rotation of the electrode with the insulating ceramic layer or melting which might result in discontinuities in the conducting layer. A method of producing a ceramic composition with a dielectric constant of greater than 2000 with a suitable temperature characteristic, which can be fired at temperatures below 1150.degree. C., would permit the use of a less costly electrode material without sacrificing capacitor performance. The dielectric ceramic compositions previously used to make multilayer capacitors at temperatures below 1150.degree. C. had dielectric constants of less than 2000, and thus were not suitable for most applications.